1. Field of the Invention
The present invention relates to a process for producing a heat developable light-sensitive composition more particularly to a process of producing a heat developable light-sensitive composition having improved light-sensitivity.
2. Description of the Prior Art
At present, the most successful light-sensitive element capable of forming a photographic image by dry processing is a heat developable light-sensitive element employing a composition comprising a silver salt of an organic acid, a small amount of a light-sensitive silver halide and a reducing agent, e.g., as described in U.S. Pat. Nos. 3,152,904 and 3,457,075. In such a light-sensitive system, the silver halide, which remains in the element after development, is not stabilized against light and allowed to be discolored by light. Nevertheless, the element provides results as if it were stabilized, because the silver halide is only used in a slight amount and most of the silver source is a white or light-colored organic silver salt which is stable and hardly blackened by light. Thus, even if a small amount of silver halide is discolored by light, the slight discoloring does not give rise to any visual difficulties since the element, on the whole, appears white or only slightly colored. The light-sensitive element is stable at normal temperature, but when it is image-wise exposed and heated usually to above about 80.degree. C., preferably above 100.degree. C., the organic silver salt oxidizing agent and the reducing agent which are present in the light-sensitive element undergo oxidation-reduction reactions due to the catalytic action of the exposed silver halide present in the vicinity thereof to form silver. By this reaction, the exposed areas of the light-sensitive layer are rapidly blackened so that contrast is formed between the exposed areas and the unexposed areas (background), i.e., an image is formed.
As processes for preparing a light-sensitive silver halide which acts as the light-sensitizing catalyst in a light-sensitive layer of this type of thermally developable light-sensitive material, processes for preparing gelatino-silver halide emulsions as are generally known in the photographic arts have been used. However, when silver halide is previously prepared and mixed with an organic silver salt as described in U.S. Pat. No. 3,152,904, the problem results that sufficient light sensitivity cannot be obtained since the organic silver salt and silver halide are not easily brought into catalytic contact with each other. On the other hand, when a halogenating agent acts on an organic silver salt to form silver halide in part of the organic silver salt as is described in U.S. Pat. No. 3,457,075, remarkably high light sensitivity can be obtained since both are in catalytic contact with each other. As a result of extensive efforts to improve this process, the invention previously discovered that when an N-halogeno compound is used as a halogenating agent, there can be obtained a thermally developable light-sensitive material having high sensitivity and undergoing lowered heat fog. In this improved process, however, there has been a limit on the light sensitivity, and it has therefore been desired to further improve light sensitivity.